1. Field of the Invention
The present application relates to a method for monitoring the hydrocarbon and water production from different production zones/sections in a hydrocarbon reservoir and detection of different phenomena such as e.g. local variations in pH, salinity, hydrocarbon composition, temperature, pressure, microorganisms, and the difference between production of formation and/or injection water from various zones/sections in wells in a hydrocarbon reservoir. The method enables both short time and long time monitoring and detection of the above mentioned phenomena. Uses of the invention are also disclosed.
2. Description of Related Art
A geological porous and permeable formation in the ground containing hydrocarbons is called a hydrocarbon reservoir. A hydrocarbon reservoir rock type consists of two elements, the matrix, which are the solid rock fragments, and the pore volume in between the rock fragments. The pore volume constitutes the porosity, xcfx86, which is the fraction of pore volumes relative to the total volume which is set equal to 1.0. For example, xcfx86=0.3 means that 30% of the total volume is pore volume and the rest of the volume (70%) is the rock matrix.
In a hydrocarbon reservoir the pore volume is generally filled with water, oil and/or gas. Due to density differences, the hydrocarbons will accumulate in the upper part of the reservoir. Water will occupy the pore spaces below the hydrocarbon zone in the reservoir. Between the hydrocarbon zone and the water zone below (the aquifer), there is a transition zone where the hydrocarbon saturation drops from close to 100% down to 0%, representing the Oil-Water-Contact, OWC.
During the production of hydrocarbons, water is normally displacing the hydrocarbon and the OWC level will rise in the reservoir. After some period of production, the water fraction of the produced fluids will increase. Generally, the fraction of water in the total volume of produced fluids increases steadily. Finally, the water fraction will be too high and the production becomes non-profitable. Often the production may come from different layers or zones of the formation cut by the well. The individual zones may have different permeability and will normally behave differently during production. It is generally an advantage to know this behavior. The gathering and analyses of information from the reservoirs during the production is called Reservoir Monitoring. The typical types of collected data are temperature, pressure, electrical resistivity, and water saturation in the reservoir close to the production well. Other types of information may be seismic maps of the whole reservoir at different stages of the hydrocarbon production.
When hydrocarbons are produced from a porous and permeable geological formation in the ground, a reservoir, the flow of hydrocarbons and water can come from different zones or layers of the reservoir. To monitor and analyze the production of different fluids, it is often important to get information about the type of fluids and the amount of the various components that flow from each zone.
A known method of examining the local flow properties in an oil and/or gas producing well is to lower a logging tool into the well. This is described e.g. in U.S. Pat. Nos. 4,861,986, 5,723,781 and 5,881,807.
To examine the local production the logging equipment lowered into the well measures the amount of oil and/or gas flowing in the well at different places along the well. Using this technique it is possible to calculate the amount of oil flowing into any region of the well. Among the disadvantages of this technique is that the production of oil and/or gas has to be wholly or partially stopped during the logging. This is a major disadvantage since one wishes to measure the local amount of oil and/or gas flowing in the well during the logging. Using this technique, it is often difficult to distinguish between the amount of oil and water in a mix flow. For a long horizontal borehole (4-6 km) it is very difficult or almost impossible to use the technique since special leading equipment is required to insert the logging probe into the horizontal wells. Furthermore, putting logging equipment into a well reduces the cross-sectional area of the producing well, resulting in a larger pressure drop. The method is also very expensive.
Another known technique in reservoir study is the application of traceable materials. Tracers are used to determine size and shape of reservoirs. U.S. Pat. Nos. 4,420,565 and 4,264,329 describe methods for following fluid flow in underground reservoirs using tracer materials. In U.S. Pat. No. 4,420,565 the depth of recovery is determined by injecting a solution containing a small amount of one or more water-soluble tracer compounds into the formation at a predetermined depth from the injection system and recovering it in the production system. The tracer compounds are then identified using gas chromatography and a flame ionization detector. U.S. Pat. No. 4,264,329 uses metal chelates as tracers, and liquid chromatography and fluorescence spectroscopy to detect the metal chelates in the produced fluid.
U.S. Pat. Nos. 3,991,827 and 4,008,763 describe a method for determination of solid particle leakage into the wellbore from different solid particle packs placed along the well using tracer particles. High concentration (50-70% by weight) of tracer particles are packed in a plurality of solid particle packs which are placed in a well to prevent solid production from the formation into the wellbore. The tracer particles used are unique for each particular pack, for example particles with different colors. If any of the solid packs is leaking during the production, the tracer particles from that pack or packs will follow the produced fluid that is analyzed at the surface to determine which pack is leaking solid particles. Using this technique, subsequent workover of the well can be limited to the pack or packs where the leaking packs are identified, rather than being directed to the whole well.
The technique described in U.S. Pat. Nos. 3,991,827 and 4,008,763 is a method for short time detection of leaking packages. Very high concentrations of tracer particles (50-70% by weight) are being used, where tracer particles are mixed with the packing particles and placed in packs around the well. There is no chemical connection between tracer particles and packing materials. The tracers will follow the flow regardless of the fluid type. The packs leaking particles will lose a large amount of tracers in a short time and therefore are not suitable for long time monitoring. In addition, migration of a large amount of solid particles such as sand and fines can wear out pipes, valves and other equipment. On the other hand, the tracers will be captured in the packs that do not leak, and therefore no information will be available from those regions. The method as it is described is useable only for examining the solid particle leakage.
U.S. Pat. No. 5,892,147 describes a procedure for determination of oil and/or gas inflow from a reservoir into a wellbore. Different radioactive isotopes are used as tracers placed in different zones along the well. The radioactive isotopes are fixed either on the outside of the transport pipe or inserted into the formation through the well casing using perforation guns. Based on the amount of the measured tracers the amount of oil and/or gas flowing into the well at each zone is calculated.
In both cases described in U.S. Pat. No. 5,892,147, the radioactive isotopes are withdrawn by the fluid as a result of the fluid flow, where the fluid could be oil, water or gas. Based on this technique it is therefore difficult to judge which zone is producing what. Fixing the radioactive isotopes on a surface of a transport pipe is limited only to those wells having a transport pipe with the arrangements required for this technique. In the other alternative the radioactive isotope particles are inserted into the formation as a load to an explosive charge which is shot through the well casing. The tracers are washed out with the first batches of fluid flow through the perforation holes since they mostly lie free on the interior surface of the perforation holes without any chemical connection to the formation. This method therefore gives some information about the initial phase of the production in each zone. In addition, the application of radioactive isotopes as tracers is expensive and requires special handling.
The present invention has been conceived to solve or at least alleviate the problems of the prior art as described above. Thus, in accordance with the present invention there is provided a method for monitoring and detection such as stated in the introduction. The method of the invention comprises
dividing regions around wells in the reservoir into a number of zones/sections, and
injecting or placing specific tracers with unique characteristics for each zone/section into the formation in these regions such that tracers are placed as integrated parts of the well completion or placed and immobilized in these regions through injection, squeeze or by other techniques, or the tracers can be immobilized or placed on a filter, a casing or constructions surrounding the well in each zone/section, the tracers being specific or introduced to give specific information from each zone/section, and the method is characterized in
chemically immobilizing/integrating the tracers in the formation or constructions/filters around the wells, the tracers (tracer carriers) being chemically intelligent and released as a function of specific events, and
detecting the tracers downstream providing information about the various zones/sections.
The regions around wells in a reservoir are divided into a number of zones/sections, and specific tracers with unique characteristics for each zone/section are placed as integrated parts of the well completion. The tracers are chemically immobilized or integrated into the formation or in/on the constructions/filters around the wells. The tracers and/or tracer carriers are chemically intelligent and released as a function of specific events. Tracers are detected downstream somewhere along the production line or on the surface, providing information about the various zones/sections in the reservoir.
Among the many advantages of the present invention is the determination of local and detailed information about the above mentioned phenomena, using a fluid intelligent tracer technique together with a relatively simple data detection system which requires no equipment down-hole during production. Therefore the well hole remains unchanged resulting in a lower pressure drop. Since there is no equipment down-hole, there is no need for equipment maintenance either. The method gives both short time and long time information which can be used, for example, for early warning, start phase detection, or long time monitoring, etc. The method uses no radioactive isotopes, or any material that is harmful to the environment.
The present invention provides a fluid intelligent and reliable tracer technique for short time and long time monitoring of different production zones/sections in oil and gas production wells and also for detection or early warning of phenomena such as water breakthrough, local variations in pH, salinity, hydrocarbon composition, temperature, pressure and microorganisms. The method enables also measurement of the difference between production of formation and/or injection water from various zones/sections in a hydrocarbon reservoir. The main advantage of this invention is that it makes it possible to collect the local detailed information from each individual zone of the reservoir, and production and/or injection wells using a fluid intelligent tracer technique. The well or wells in a reservoir are divided into a certain number of separate zones/sections equipped with proper tracers of unique characteristics for each zone/section as illustrated in FIG. 1. The tracers are placed as integrated parts of the well completion chemically immobilized or integrated into the formation and/or in/on the constructions or filters around the wells. The tracers can also be placed in small chemically intelligent packages located around the well in the reservoir and/or in the constructions or filters around the wells (FIGS. 2-8). The tracers can be immobilized or placed on the filter or the casing around the well in each zone/section. The tracers and/or tracer carriers are chemically intelligent and released as a function of specific events. Tracers are detected downstream somewhere along the production line or on the surface. The collected information can be used to study the local flow conditions in each zone/section and to describe the reservoir behavior during production. Such information includes for example production rate, location and the type of produced fluids, e.g. how much oil, water or gas is produced and from where in the reservoir they come. The method can be used to study the reservoir dynamics behavior in the neighborhood of the production and injection wells. The method can be used to detect the phenomena such as water breakthrough, variations in pH, salinity, hydrocarbon composition, temperature, pressure and microorganisms in each well section/zone. Using intelligent tracers it is also possible to distinguish between produced injection water and formation water from each zone/section. The technique can be also used as a local alarm- or early warning system for both quantitative and qualitative detection of the appearance of phenomena such as water breakthrough, variations in pH, salinity, hydrocarbon composition, temperature, pressure, and microorganisms. The invention can in principle also be used for other production processes and fluid transport facilities.